Polar Ice Loss and Sinking Land Push Coastal Flood Risks Higher

WASHINGTON — More than 12 trillion tons of ice have melted from Greenland and Antarctica over the last half-century, while localized land subsidence is compounding the threat to global coastlines. The loss of 12 trillion metric tons of polar ice between 1972 and 2020 contributed approximately one inch to global mean sea level.

According to research published in the peer-reviewed journal *Scientific Data*, every third of an inch of sea-level rise exposes an additional 2 to 3 million people worldwide to annual coastal flooding events. Millions of coastal residents are already facing heightened flood risks from the inch of rise attributed to polar melt alone. Oceanographers and climate scientists warn that the marginal impact on coastal flooding is non-linear and immediate, even though one inch may appear minor.

A separate study in the journal *Nature* examined the interaction between rising oceans and vertical land motion across 32 major U.S. coastal cities. It found that the combined effects of rising seas and sinking land, known as subsidence, could expose between 176,000 and 518,000 people to severe flooding by 2050.

The polar findings were compiled under the Ice Sheet Mass Balance Inter-comparison Exercise (IMBIE), an international collaboration supported by the National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA). Scientists reconciled data from multiple satellite missions, using altimetry to measure ice height and gravimetry, through missions such as GRACE (Gravity Recovery and Climate Experiment), to measure changes in ice mass.

Greenland and Antarctic ice sheets were relatively stable through the 1970s, 1980s, and 1990s, but runoff and ice discharge accelerated dramatically during the 2010s. “There is a clear trend that the ice sheets are losing more mass and it has been increasing from decade to decade, especially if you look at the ice being discharged to the ocean,” said Ines Otosaka, a researcher at the University of Leeds and the lead author of the study. “And that’s quite clear that there’s a link to climate change.”

Subsidence occurs when the Earth’s surface settles or sinks. Along the U.S. Atlantic and Gulf coasts, it results from natural geological settling, sediment compaction, and human activities including groundwater extraction and oil and gas withdrawal. When land sinks while the sea rises, relative sea-level rise is effectively doubled in some regions.

The physical response of massive continental glaciers also trails atmospheric warming. “That’s something to be concerned about as it shows that the changes are tracking decades behind global warming,” said study co-author Andrew Shepherd, a professor of Earth observation. “Which means we can expect them to continue for decades more — even if we are able to stop heating the planet.”

In 2022, a federal task force led by the National Oceanic and Atmospheric Administration (NOAA), NASA, and the Environmental Protection Agency (EPA) released updated sea-level rise projections. The interagency report outlined five distinct scenarios for U.S. coastlines through 2100, based on varying greenhouse gas emission pathways, and projected an average of 10 to 12 inches of sea-level rise between 2020 and 2050. That 30-year increase matches the total rise measured from 1920 to 2020.

The potential for rapid, non-linear collapse of the West Antarctic Ice Sheet remains the primary source of uncertainty in the projections and could push sea levels significantly higher than moderate estimates. In South Florida, Miami has some of the highest projected exposure to flooding on the Atlantic coast, partly because it sits on highly porous limestone bedrock. Rising seas can push saltwater upward through the underground rock, making traditional defenses such as seawalls and dikes ineffective and threatening the Biscayne Aquifer, the primary drinking-water source for millions of residents, through saltwater intrusion.

Much of New Orleans already sits below sea level and is protected by levees, floodwalls, and pumps within the Hurricane and Storm Damage Risk Reduction System (HSDRRS). The Mississippi River Delta, however, is sinking rapidly because upstream levees prevent natural silt replenishment, creating sediment starvation, while historical oil extraction has added to the problem. Galveston, Texas; Mobile, Alabama; and Naples, Florida, face similar pressures from rapid subsidence and intensifying tropical storm surges.

As the changes continue, policymakers have shifted their debate from whether sea levels are rising to how quickly municipal governments must adapt. The compounding hazards are driving up municipal adaptation costs, restructuring the coastal real estate market, and straining the National Flood Insurance Program (NFIP) as flood zones expand further inland.